Naval security forces killed that man after he fatally shot the sailor who came to the aid of the petty officer on watch, the Navy said. The identities of the two who died have not been released.

"Information about our sailor will come at the appropriate time, and today is not that day," said Capt. Robert Clark, Naval Station Norfolk's commanding officer. He asked for privacy for the sailor's family.

The incident occurred at about 11:20 p.m. ET Monday at Pier 1 aboard USS Mahan, base spokeswoman Terri Davis said. She didn't know whether the suspect had permission to be on board, but he was authorized to be on base.

The civilian suspect approached the destroyer's quarterdeck, and the ship's security personnel confronted him, Navy officials say. A struggle occurred, and the suspect was able to disarm the petty officer.

No other injuries were reported.

"I want to be clear: The suspect did not have his own weapon," Clark said.

To get on the Navy base, civilians must be escorted or have identification that allows them to be there. Authorized civilians can include Department of Defense employees, contractors and military family members.

Each base entrance is guarded, and motorists present IDs. Inspections are rare. All 13 piers have additional security forces. As part of ongoing security efforts, handheld ID scanners were put in place this year at Navy bases in the region, including the Norfolk station.

The suspect had what the Navy calls a Transportation Worker Identification Credential, which allows maritime workers unescorted access to secure areas of port facilities and vessels, he said. To obtain that credential, a person must provide personal information, give biometric data such as fingerprints, sit for a digital photo and pass a Transportation Security Administration security threat assessment, according to the Navy Sealift Command website.

The cards are commonly issued to truck drivers, employees of the Navy Military Sealift Command, merchant mariners and other employees who work at a commercial port. The program was created after Sept. 11, 2001, as a way to tighten security at commercial ports and other sensitive civilian areas that could be a high-value target for terrorists.

Military Sealift Command hires civilian mariners to crew its ships, including the hospital ship USNS Comfort, which uses the same pier as the Mahan. Clark said it was unclear what exactly the civilian's job was or when he worked on the base.

"To get on the pier is tight security," retired Navy Capt. Joe Bouchard said. "To get onto the ships, there's tight security. So an incident like this happening is really a shock."

Members of Congress have been skeptical of the value of the TWIC cards in recent years. The high-tech biometric data that is stored in the card is rarely accessed because the card is not scanned as it was designed to be.

A number of memos and advisories from the Department of the Navy show that TWIC holders can get unaccompanied access to a base if they present their card for inspection along with a valid bill of lading or other form of verifiable documentation.

"It's going to make us look at our procedures to make sure we're doing everything we need to do," Clark said. "I think we're making improvements every day to our security to make sure we stay safe."

After the incident, the base briefly was put on lockdown for about 45 minutes as a precaution. With the exception of Pier 1, the first of 13 main piers, operations have returned to normal at Naval Station Norfolk, Davis said.

"We'll find out what happened, and we'll prevent that from occurring again," Adm. Bill Gortney, commander of U.S. Fleet Forces Command based in Norfolk, said to a group of sailors gathered for a training session unrelated to the shooting.

The Mahan, commissioned in 1998, has a crew of nearly 300. In September, it returned to Norfolk after a deployment of more than eight months that included being positioned in the eastern Mediterranean for a potential strike against Syria.

The Norfolk base covers more than 6,000 acres and is the home port for 64 ships, according to information the Navy provided in February. About 46,000 military members and 21,000 civilian government employees and contractors are assigned to the base and its ships.

The shooting comes a week after the Navy began to study recommendations following September's mass killing at Washington Navy Yard, about 150 miles away, in which a contractor and former Navy reservist killed 12 civilian workers before being shot to death.

The shooting also comes about a month after the Navy had anti-terrorism and force protection exercises around the world, including an active-shooter drill at Naval Station Norfolk.